It has long been known to log open boreholes to measure acoustic wave energy traveling through rock formations surrounding the borehole. Borehole logging tools have been used for this purpose which normally comprise a sound source, or acoustic transmitter, and one or more receivers disposed at spaced-apart distances from the transmitter. By measuring the travel time of such acoustic energy between the transmitter and receivers it is possible to determine the nature of properties of the surrounding rock formations.
Typically such borehole logging tools have provided acoustic energy waves in the form of compressional waves, shear waves, tube waves and normal mode or pseudo-Rayleigh waves. Various signal timing and wave front analysis methods have been utilized for distinguishing between these various waves received at a given receiver. For descriptions of various logging techniques for collecting and analyzing acoustic energy waves in the borehole reference may be made to U.S. Pat. Nos. 3,333,238 to Caldwell; 3,362,011 to Zemanek, Re. 24,446 to Summers; and 4,383,308 to Caldwell.